High pressure fuel pumps known in the art have a housing provided with a bore defining a compression chamber and, a cylindrical piston reciprocally translating within the bore so that fuel in the compression chamber is pressurized. While the majority of the pressurized fuel exits via an outlet orifice, a small quantity of pressurized fuel leaks through the functional clearance that is between the bore and the piston. In operation, when fuel is under high pressure, the clearance increases allowing more important fuel leak.